Hao, in: Ye, J. Heidemann, D. Estrin, in: Zhao, M. Ma, C. Miao, T. Computer Communications Vol. Nikolaos, J. Dimitriosergados, D. Dimitriose, D. Ad Hoc Networks Vol. Mao, Z. Wu, X. Xiao, B. Zheng, Z. Yan, C.
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Cited by. Related Articles. Paper Title Pages. In recent years, tube hydroforming has been applied in automobile and airplane industries, to decrease weight. In general, the determination of internal pressure path is a key factor in improving the formability in tube hydroforming. One of the main disadvantages of AS-MAC is the inefficiency in packet broadcasting, since it has to transmit every broadcast packet once per each neighbor. A protocol that overcomes the weaknesses of AS-MAC is AS2-MAC [ 3 ], which is based on an efficient setting of the node's duty-cycle as a function of the transmission times of the neighbor nodes.
In a duty-cycle period, each node wakes up once to transmit and N times to receive, where N is the number of neighbors, while it remains in sleep mode for the rest of the time. In order to improve the performance of MAC solutions without burdening the limited resources of WNS nodes, several works in the literature exploit the combined use of software algorithms and hardware solutions.
In [ 17 ], a protocol that uses a directional antenna and a busy tone to ensure the energy saving is proposed. Messages that need to be sent in broadcast force the antenna to work in omnidirectional mode, whereas unicast messages are sent in one direction using the information stored in the cache table. Instead, the Direction Antenna at Sink DAaS protocol [ 18 ] extends the lifetime of the network by increasing the transmission range of the sink and by scheduling wake-up and sleep times for nodes according to the SMAC protocol.
In particular, by changing the transmission range of the sink, this protocol distributes more fairly the power consumption among nodes communicating directly with the sink. In particular, the protocol achieves time-space synchronization in presence of mobile nodes and allows the management of the smart antenna adapting the Radio Frequency parameters in accordance with the characteristics of the communication link.
This solution results in a decrease of radiated power of about N times compared to a transmission in omnidirectional mode, where N is the number of antenna sectors. It significantly reduces the nodes' power consumption by exploiting AS2-MAC protocol and directional communications. On the one hand, the scheduling algorithm of AS2-MAC reduces the number of collisions in the network and faces phenomena such as the hidden node problem, the idle listening, and the overhearing. On the other hand, the switched-beam antenna further reduces the power consumption by focusing communications only towards the proper sector.
The basic idea of the defined protocol is to ensure smart awakenings; that is, nodes wake up only when they actually have data to send or receive and only for the needed time. Moreover, by exploiting the switched-beam antenna, the transmission power is focused only on the desired direction in order to keep the power consumption as low as possible. More in detail, the proposed MAC schema consists of two main parts: The network setup procedure, in turn, is characterized by three phases: During transmission time announcement phase, each node chooses its transmission time i.
During the first phase, every node detects where its neighbors are located, that is, the antenna sector useful to reach each neighbor. Finally, during next hop announcement , every node notifies the network about the identity of its next hop. During the steady state, the information gathered through network start-up is used to optimize both outcoming and incoming transmissions.
Of course, if a transmission occurs, only the needed antenna sectors are exploited. For the sake of clearness, before describing the new scheduler in detail, some parameters used in the discussion are introduced below. It is the same for every node and it is preconfigured Figure 1. WakeTime is the maximum time interval in seconds for transmitting local data or for receiving packets from neighboring nodes Figure 1. SleepTime is the minimum time interval in seconds that each node spends in sleep mode Figure 1.
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Announce Packet Pk t ANN is a signaling packet used by each node to advertise its presence; it contains the time interval between the current time and the next awakening time chosen for transmission. Reservation Packet Pk t RSV is the packet used by a node to reserve the channel for the sector discovery phase. Discovery Packet Pk t DSC is a unicast packet used to identify which neighbors are reachable through each antenna sector. Each table entry contains the following information: As said in the previous section, the first step of the network start-up consists in the exchange of information about the transmission time.
However, before being stored, the information on the transmission time of the neighbor must be validated; that is, the time chosen by the new neighbor has not to overlap with the transmission intervals of the other neighboring nodes already stored into its W TBL. If the validation succeeds, the transmission time of the neighbor is converted into offset to be consistent with the time view of the receiving node and then it is stored with the other offsets in ascending order.
Otherwise, if some overlaps occur, the receiving node sends a Pk t ALERT to its neighbor, specifying the overlapping interval. In such a case, the neighbor stores the received information into its W TBL and chooses a new transmission time. This mechanism also reduces one of the main problems that afflict ad hoc networks, that is, the hidden node problem: If the new node cannot find a valid transmission time, that is, the network is full, it broadcasts a Pk t FULL and turns off the radio.
An Energy Efficient Cross-Layer Protocol for Wireless Sensor Networks
On the reception of such a message, all the neighbors delete the entry eventually stored for that node. It is worth noting that the probability of a missed join can be controlled through an accurate tuning of network parameters, first of all the duty-cycle period T 0 and the WakeTime interval. Indeed, if the duty-cycle period is increased and the WakeTime is decreased, then a new node has more chances to find a free time slot for transmission and, in addition, all the nodes save more energy. However, under a heavy traffic load, this setup's conditions could lead to a rapid worsening of the network performance due to an increase of the buffer overflow events i.
Figure 2 shows a flow chart that describes this first step of the network setup. Figure 2 Flow chart of the transmission time announcement phase. Let us now describe how a node chooses its own transmission time. As said, the timeline is divided into fixed slot of T 0 seconds and each node has to choose its own transmission time as a random value in this interval, also taking into account the choices done by its neighbors.
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This separation in time among transmissions of neighboring nodes leads to reduce the contention for the channel access. If the W TBL is not empty, then the node tries to set its own transmission time to a value different from those of its neighbors, in order to avoid collisions due to simultaneous transmissions. Note that the node also checks the time intervals: In order to maximize the probability that all its neighbors receive the message, a node sends the Pk t ANN three times.
During the sector discovery phase, each node learns where the neighbors previously stored in W TBL are located; that is, it detects the proper antenna sector for communicating with each neighbor. This mechanism allows obtaining a reduction of power consumption during both transmission and reception phase.
Indeed, in both cases, the node enables only the antenna sector useful to communicate with the desired neighbor. A node performs the localization phase by broadcasting, during its transmission time, a Pkt RSV simultaneously in all directions. Through this packet, it communicates the total amount of time needed for the discovery phase, which is equal to the number of neighbors multiplied by the number of sectors and by the time interval required for sending a packet and receiving the corresponding acknowledgment.
After the channel reservation, a node sends a unicast Pkt DSC on each antenna sector and waits a response for a short time period.
Energy Efficient Cross-Layer Transmission Model for Mobile Wireless Sensor Networks
In fact, since the radiation lobes of adjacent antenna sectors may overlap, the discovering node may receive more responses relating to different sectors from the same neighbor; in this case, it stores the sector corresponding to the response with the highest RSSI value. Figure 3 summarizes the sector discovery procedure.
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The last step of the network start-up is the next hop announcement, which is carried out after the convergence of the routing algorithm. So, knowing in advance the previous hops allows keeping OFF the radio transceiver if the incoming communications have not to be received. To carry out this mechanism, upon the convergence of the routing protocol, the network layer of each node communicates to the MAC layer the identity of the next hop.
Of course, the next hope announcement is triggered every time the routing algorithm is fired due to topology changes. In this phase, each node knows i the transmission times and ii sectors of its neighbors, as well as iii the identity of its previous hops.
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